Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Commitments And Contingencies [Line Items]
Additional successive terms extendable at the option	4 years
Period of extendable lease term, in years	5 years
Lease term, description	Less than one year
Sale leaseback transaction, gain recognized over lease term	$ 66.6	$ 68.3
Loan commitments	1,700onb_LoanCommitments	1,584onb_LoanCommitments
Fixed rate loan commitment	1,619
Floating rate loan commitment	80.9
Standby letters of credit	60.0	65.3
Loan commitments fixed interest rate, minimum	0.00%
Loan commitments fixed interest rate, maximum	21.00%
Allowance for unfunded loan commitments	2.8onb_AllowanceForUnfundedLoanCommitments	4.4onb_AllowanceForUnfundedLoanCommitments
Extended credit	16.0onb_CreditExtended	13.0onb_CreditExtended
Credit extensions with collateral	$ 14.5onb_CreditExtensionsWithCollateral	$ 11.5onb_CreditExtensionsWithCollateral
Minimum [Member]
Commitments And Contingencies [Line Items]
Lease Term	1 year
Minimum [Member] \| Indiana Community Bancorp [Member]
Commitments And Contingencies [Line Items]
Lease Term	5 years
Maximum [Member]
Commitments And Contingencies [Line Items]
Lease Term	24 years
Maximum [Member] \| Indiana Community Bancorp [Member]
Commitments And Contingencies [Line Items]
Lease Term	24 years

Allowance for unfunded loan commitments.

No definition available.

Name:	onb_AllowanceForUnfundedLoanCommitments
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	onb_CommitmentsAndContingenciesLineItems
Namespace Prefix:	onb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of credit extended with unaffiliated banks related to letter of credit commitments issued on behalf of our clients.

+ References

No definition available.

+ Details

Name:	onb_CreditExtended
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of collateral provided to secure credit extended with unaffiliated banks related to letter of credit commitments issued on behalf of our clients.

+ References

No definition available.

+ Details

Name:	onb_CreditExtensionsWithCollateral
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Extendable lease terms maximum number.

+ References

No definition available.

+ Details

Name:	onb_ExtendableLeaseTermsMaximumNumber
Namespace Prefix:	onb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Leases term.

+ References

No definition available.

+ Details

Name:	onb_LeasesTerm
Namespace Prefix:	onb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Loan commitments.

+ References

No definition available.

+ Details

Name:	onb_LoanCommitments
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Loan commitments fixed interest rate maximum.

+ References

No definition available.

+ Details

Name:	onb_LoanCommitmentsFixedInterestRateMaximum
Namespace Prefix:	onb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Loan commitments fixed interest rate minimum.

+ References

No definition available.

+ Details

Name:	onb_LoanCommitmentsFixedInterestRateMinimum
Namespace Prefix:	onb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Notional amount of standby letters of credit.

+ References

No definition available.

+ Details

Name:	onb_NotionalAmountOfStandbyLettersOfCredit
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Option to extend lease term.

+ References

No definition available.

+ Details

Name:	onb_OptionToExtendLeaseTerm
Namespace Prefix:	onb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Original lease term description.

+ References

No definition available.

+ Details

Name:	onb_OriginalLeaseTermDescription
Namespace Prefix:	onb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of deferred gains remaining associated with a prior sale leaseback transaction.

+ References

No definition available.

+ Details

Name:	onb_SaleLeasebackTransactionGainRecognizedOverLeaseTerm
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the amount of cash required to fully fund agreements to provide advances to borrowers and lessees at stated interest rates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 825

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6480020&loc=d3e61082-112788

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 825

-Section 50

-Paragraph 1

-Subparagraph (b)(2)

-URI http://asc.fasb.org/extlink&oid=6480020&loc=d3e61044-112788

+ Details

Name:	us-gaap_LoansAndLeasesReceivableCommitmentsFixedRates
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Reflects the amount of cash required to fully fund agreements to provide advances to borrowers and lessees at interest rates that increase or decrease depending on certain facts or circumstances.

+ References

+ Details

Name:	us-gaap_LoansAndLeasesReceivableCommitmentsVariableRates
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: